After fabrication, and after various steps during processing, samples such as glass panels, semiconductor wafers, and other materials are often tested for a variety of physical, electrical, mechanical and chemical properties. One such test that is often performed on sheet samples is to measure the sample's sheet conductance. In one example, the sheet conductance (and/or resistance) of a semiconductor wafer is measured to ensure the absence of undesirable sheet resistance variations in epitaxial or ion implanted and annealed wafers, prior to adding expensive subsequent processing steps required to produce integrated circuits.
It is known that sheet conductance may be measured as a proportional DC voltage. Knowledge of the sheet conductance of a material sample is important, because it allows for the early identification of defects in the material prior to undertaking subsequent expensive process steps that are required to produce modern integrated circuit devices. If the sample does not conform to a known sheet conductance profile, then it may be presumed that a defect is present in the wafer or in a portion of the wafer, and the wafer may be scrapped, or further processing steps may be performed taking into account the defect(s).
Current methods and systems for measuring sheet resistance of a sample typically use a two coil configuration, with one coil positioned on opposite sides of the sample under test. The benefit of such two-coil configurations is that they provide two magnetic fields that penetrate the sample or samples, and thus the sample is subjected to a relatively evenly distributed field, which is important for purposes of obtaining an accurate sheet resistance measurement. The disadvantages of such two-coil systems are that the two coils must be placed close enough to each other in order for the eddy current process to be effective, which thus limits the overall thickness of the sample being measured. Additionally, for so-called “series aiding” two-coil systems, measurement of large-diameter samples requires longer lead lengths between the coils, which can result in problems with frequency and loss, thus reducing the stability of the circuit that generates the magnetic field. As a result, the measurements obtained may have a less than desired accuracy.
Furthermore, it may be difficult and time consuming to move large samples in and out of the opposing coils, and to position them appropriately when a plurality of measurement locations are involved.
Thus, there is a need for an improved system for measuring sheet conductance of material samples. Specifically, there is a need for a sheet conductance testing device that is easily movable and positionable with respect to the sample to enable testing without extensive handling and positioning of the sample, thus eliminating the need for a sample to be moved between a pair of coils, and thereby enabling practical testing of larger and thicker samples.
There is also a need for a device that can be used to test materials other than traditional semiconductor and/or flat panel materials. For example, it would be desirable to provide a non-invasive system and technique for performing conductance measurements in aid of a variety of diagnostic medical analyses, such as the monitoring of blood circulation and oxygenation of human tissue. Prior art techniques using ultrasound are capable only of showing, for example, whether a blood vessel is allowing satisfactory circulation, but still may not show whether oxygen from the circulated blood is being adequately moved to the surrounding tissue, to thereby provide a gauge of the general health of the tissue. Thus it would be advantageous to provide a system that can identify situations in which blood circulation is satisfactory, but nonetheless where tissue may still be dying. Such systems would advantageously provide information to medical personnel that is not currently available through non-invasive techniques.